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Dungeons and Deltas

I spent much of the past week in New Orleans and Nashville, indulging in live music and warm weather. Obviously, the best part of travelling is geology, and although my only exposure to ‘rocks’ involved an afternoon on the beach, the Mississippi River delta on which New Orleans is built is quite a story in and of itself.

At 5,970 km (3,710 miles), the Mississippi-Missouri River is the fourth-longest river in the world, following the Nile, the Amazon, and the Yangtze. The Mississippi delta is a textbook ‘bird’s foot’, with several well-defined channels forming ‘fingers’ that extend out into the Gulf of Mexico.

A satellite image of the Mississippi River delta. The green area is covered by wetlands and salt marshes

What is a delta exactly? A delta is a fan-shaped deposit formed when a river or stream enters a larger body of water such as a lake or ocean. Picture, if you will, a freeway exit during rush hour. As the larger, faster highway funnels into the smaller, slower street, cars accumulate and a traffic jam forms. In the same way, the amount of sediment that a water body can carry is proportional to its ‘energy’ or speed of flow. Fast-flowing rivers can carry gravel (as well as anything smaller) and roll rocks along their beds. Slower streams can carry sand and silt. In a standing water body such as a lake, all of these particles settle out. Thus, as the Mississippi enters the Gulf of Mexico, its sediments are deposited forming a delta. Over thousands of years, the Mississippi River delta has advanced into the Gulf, creating prime real estate (read: swamps and marshes) in its wake.

Evolution of Mississippi River delta over the past 6000 years

A coastal delta is subject to a constant war of elements. Just as a delta grows through sediment accumulation, it shrinks as waves erode it away. In an active delta, the growth is greater than the ‘wear and tear’, allowing it to advance over time. If a delta’s sediment supply is cut off, however, it will begin to retreat. As rivers naturally evolve and change course, old deltas are abandoned and new ones formed.

Water tends to follow the path of least resistance, constantly looking for shorter, steeper paths to a given destination. Imagine that you’re back in that same traffic jam as before. There’s always one driver who decides to cheat and pass everyone on the shoulder. Sometimes, others follow suit. In the case of the Mississippi River, this ‘short-circuiting’ process occurs approximately every thousand years and leads to the formation of a new delta lobe.

A reconstruction of past delta lobes from geological data

Another example of a natural ‘short-circuit’. Here you can see a bend in the river that has been cut off from the main channel as the river has taken a shorter path

The latest cycle has seen the increasing loss of water from the old channel to the Atchafalaya River, which branches off a good ways northwest of New Orleans. Were this process to continue, the channel would eventually shift completely, abandoning the existing delta. This poses two major issues for infrastructure. First, New Orleans and surrounding towns would essentially be cut off from the river. Second, the expansion of the new channel would fuel flooding in neighboring areas.

In the mid-20th century, the US Army Corps of Engineers undertook a massive effort to control the river channel and prevent the it from shifting completely. Through the construction of levees and floodgates, the Mississippi River was confined to its existing channel, but at a cost. Without the ability to flood or migrate, much of its sediment load is now dumped out to sea, which has caused the delta to shrink in the decades since. Together with the construction of channels and shipping routes, this has also damaged the extensive array of wetlands that crisscrosses the region, although in the past decade, new measures have been implemented to mitigate this decline.

Until next time, be warned: you may have just sustained a lethal dose of mostly harmlessscience.

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